1. Field of the Invention
The present invention relates to a reflection-reducing film having excellent liquid resistance and, more specifically, to a reflection-reducing film formed by coating the upper surface of a light transmitting substrate to impart liquid resistance against liquids such as tap water, sweat, etc.
2. Description of the Prior Art
A reflection-reducing film has been extensively used for the panels that serve as optical display surfaces in the devices such as CRT, LCD and plasma display. These reflection-reducing films are generally formed by vacuum vaporization, method, sputtering method or wet coating method.
There has also been known a reflection-reducing film comprising plural layers formed on a plastic substrate. For example, Japanese Patent Laid-Open No. 288202/1997 discloses a reflection-reducing film comprising a plastic substrate having light-transmitting property, a layer of a high refractive index applied onto the substrate, a reflection-preventing layer applied onto the layer of a high refractive index, and a coated layer formed on the reflection-reducing film. In this reflection-reducing film, the layer of a high refractive index comprises, as chief components, a metal alkoxide, a colloidal metal oxide and/or a metal halide, the reflection-reducing layer comprises an amorphous fluorine-contained resin having a refractive index (nD) of not larger than 1.36, and the coated layer comprises an organic polysiloxane as a chief component and a fluorine-contained material having a surface-activating ability. This prior art teaches that the reflection-reducing film is excellent in abrasion resistance, scratch resistance, adhesiveness and light transmission property.
The known reflection-reducing film is satisfactory from the standpoint of preventing reflection but is not still satisfactory from the standpoint of durability and, particularly, liquid resistance, and is subject to be deteriorated by various liquids while it is being used for extended periods of time resulting in the occurrence of a so-called film peeling.
That is, the first layer (surface layer) of the reflection-reducing film contains silica (SiO2) as a main component to satisfy the requirement of hardness but tends to be dissolved upon contact with an acid or an alkali. The reflection-reducing film is corroded by, for example, sodium hypochlorite contained in the tap water, by acid or alkali contained in the sweat, and by sodium ions contained in the spray of seawater.
In the case of cellular phones, in practice, a problem is arousing in that the reflection-reducing film is easily peeled off the liquid crystal display unit due to the adhesion of sweat from the ear.
It is therefore an object of the present invention to provide a reflection-reducing film which can be easily formed on the light transmitting substrate by applying a multi-layer coating thereon and which exhibits excellent liquid resistance.
Another object of the present invention is to provide a reflection-reducing film having a non-glaring (glare-preventing) function capable of effectively blurring the transmitted image only.
A further object of the present invention is to provide a reflection-reducing film featuring excellent light resistance and capable of effectively preventing the film from being peeled even after used for extended periods of time.
According to the present invention, there is provided a reflection-reducing film formed by being applied onto a surface of a substrate having light transmitting property, and including a base layer formed on the surface of the substrate and a surface layer formed on an upper side of the base layer, wherein:
the base layer has a thickness of from 50 to 200 nm; and
the surface layer is formed of a hydrolyzed product of a silane compound represented by the following formula (I),
Rnxe2x80x94Si(X)4-nxe2x80x83xe2x80x83(I) 
wherein R is a substituted or unsubstituted alkyl group or a substituted or unsubstituted alkenyl group, X is a hydrolyzable group, and n is a number of 1 or 2,
a metal chelate compound and a silica sol, and has a thickness of from 50 to 200 nm.
The reflection-reducing film of the present invention includes at least two layers, i.e., a base layer formed on the surface of the substrate and a surface layer formed on the upper side of the base layer. However, an intermediate layer having a thickness of 50 to 200 nm may be provided between the base layer and the surface layer.
In the present invention, a feature resides in that the surface layer contains a hydrolyzed product of a silane compound of the above formula (I) and a metal chelate compound. This imparts liquid resistance to the reflection-reducing film that is formed by the wet method.
Reference should be made to Examples described later. A reflection-reducing film (Comparative Example 1) having a surface layer which comprises, as a chief component, the hydrolysis of an ethyl silicate (tetraethoxysilane) dissolves when it is immersed in the tap water, saline solution or artificial sweat solution.
On the other hand, the reflection-reducing films (Examples 1 to 3) having a surface layer 1 formed of a hydrolyzed product of a compound of the formula (I) and a metal chelate compound, do not at all dissolve even when they are immersed in these liquids, and exhibit markedly excellent liquid resistance.
Besides, the surface layer has a small refractive index, which is a markedly excellent property.
It is considered that the improvement in the liquid resistance according to the present invention stems from a chemical structure of the silane compound (silane coupling agent) of the formula (I). That is, the compound of the formula (I) has two to three hydrolyzable groups (X) bonded to a silicon atom, and one to two organic groups (substituted or unsubstituted alkyl groups or alkenyl groups R) bonded to the silicon atom.
When the silane coupling agent is hydrolyzed, two to three hydrolyzable groups undergo the hydrolysis to form a two-dimensional or three-dimensional siloxane bond. It is here considered that one or two organic groups (R) remain, and the combination of these organic groups (R) and the siloxane bond contribute to improving the liquid resistance. Further, the metal chelate compound exists on the surface layer of the reflection-reducing film of the invention and, hence, a crosslinked structure is introduced into the surface layer to further improve the liquid resistance of the surface layer.
Further, the organic groups (R) in the silane coupling agent improves the adhesion or intimate adhesiveness to the intermediate layer or to the base layer, improving the degree of coupling in the interface to the silica fine particles blended in the surface layer. Therefore, the surface layer formed of the silane coupling agent used in the invention exhibits excellent liquid resistance against the tap water, artificial sweat solution and saline solution.
In the present invention, it is desired that the silica sol contained in the surface layer has a refractive index of as low as not larger than 1.44. Upon containing the silica sol having an average particle diameter of from 5 to 200 nm, in particular, the refractive index of the surface layer can be decreased to be not larger than 1.44.
In the reflection-reducing film of the three-layer constitution having an intermediate layer provided between the surface layer and the base layer, it is desired that the intermediate layer contains a metal alkoxide, a hydrolyzed product of a silane coupling agent and a silica sol.
In the present invention, the intermediate layer is formed of the metal alkoxide and the hydrolyzed product of the silane coupling agent. Therefore, the intermediate layer exhibits an enhanced refractive index, improved film strength and improved liquid resistance. Further, the intermediate layer blended with the silica sol exhibits improved adhesiveness (bonding force) to the surface layer and to the intermediate layer.
In the reflection-reducing film of the invention, it is desired that the surface layer has a refractive index smaller than that of the base layer, e.g., has a refractive index of as low as not larger than 1.44. In the reflection-reducing film of the three-layer structure having the intermediate layer, further, it is desired that the intermediate layer has a refractive index of as high as not smaller than 1.75 and the base layer has a refractive index that lies between those of the surface layer and the intermediate layer.
Due to the gradient of the refractive indexes compounded by the thicknesses of the above-mentioned layers (each layer has a thickness of from 50 to 200 nm), both a high light transmission factor (usually, not smaller than 97%) and a low light reflecting property (usually, not larger than 3%) are obtained.
In the present invention, further, it is desired that the base layer is formed on the surface of the substrate through the hydrolysis of the silane coupling agent from the standpoint of obtaining excellent adhesiveness to the substrate and preventing photocatalytic action.
In the reflection-reducing film having the three-layer structure including the intermediate layer, further, it is desired that fine particles having an average particle diameter of from 50 to 1000 nm and, particularly, from 100 to 500 nm are dispersed in the base layer and in the surface layer. The reflection-reducing film having layers in which fine particles are dispersed exhibits excellent non-glare function, does not blur the transmitted image but effectively blurs the reflected image only, making it possible to strikingly improve easiness of viewing the optical display surface.
In the reflection-reducing film of the three-layer structure, further, the refractive index of the intermediate layer is improved usually by blending the intermediate layer with a titanium dioxide (TiO2) component. According to the study conducted by the present inventors, however, it was learned that the titanium dioxide component and, particularly, the anatase-type titanium dioxide causes the reflection-reducing film to be deteriorated by light.
The titanium dioxide and, particularly, the anatase-type titanium dioxide is usually called optical catalyst or optically reactive semiconductor, forms electron-positive hole pairs upon the irradiation with ultraviolet rays of wavelengths of chiefly not longer than 400 nm to trigger a photocatalytic reaction.
If described in connection with this point, a photo-semiconductor metal oxide as represented by TiO2 knocks out electrons from a band of valence electrons into a conduction band (excited) when it is irradiated with light having energy larger than a band gap (energy gap between the band of valence electrons and the conduction band) thereof, and particles (positive holes) having positive electric charge are formed in the band of valence electrons. Electrons (exe2x88x92) and positive holes (p+) partly migrate onto the surfaces of the oxide particles, the electrons taking part in the reducing reaction and the positive holes taking part in the oxidation reaction as represented by the following formulas,
semiconductor+hxcexdxe2x86x92exe2x88x92+p+xe2x80x83xe2x80x83(1) 
exe2x88x92+O2xe2x86x92O2xe2x88x92xe2x80x83xe2x80x83(2) 
p++H2Oxe2x86x92OH+H+xe2x80x83xe2x80x83(3) 
p++OHxe2x88x92xe2x86x92OHxe2x80x83xe2x80x83(4) 
A superoxide (O2xe2x88x92) formed by the above reaction (2) and hydroxy radicals (OH) formed by the reactions (3) and (4), exhibit strong oxidizing force and work to oxidize organic matters. It is therefore considered that the film is deteriorated due to this oxidation.
In practice, a hydrolyzed product of a titanium alkoxide is usually used for the formation of a layer having a high refractive index in the reflection-reducing film. Here, however, the anatase-type titanium dioxide having large optical activity is easily formed by the hydrolysis of the titanium alkoxide and, besides, it is believed that the hydrolyzed product of the titanium alkoxide is not cured sufficiently and promotes the optical activity.
That is, when the intermediate layer is blended with the titanium dioxide component as described above, the base layer on the lower side tends to be deteriorated by optical oxidation and, as a result, the film tends to be peeled off. In the present invention, the base layer interposed between the intermediate layer and the transparent substrate does not substantially contain organic resin component, in order to prevent optical oxidation of the base layer and, hence, to effectively prevent the film from being peeled off by light.